The belt used to secure and carry a katana is called an "obi." The obi is a wide, long sash or belt that is an integral part of traditional Japanese attire, including the kimono and other garments. In the context of a katana, the obi serves as the belt used to secure the sword and its scabbard (saya) around the waist.

The obi is typically made from a sturdy fabric, such as silk or cotton, and can be plain or elaborately decorated. It is wrapped around the waist and tied in a specific manner to ensure a secure fit for carrying the katana. The tying style may vary depending on the specific school or style of martial arts or the personal preference of the wearer.

The obi provides support and stability for the katana, keeping it firmly in place and preventing it from shifting or falling out of the scabbard during movement. It is an essential component of the traditional attire for wearing a katana and is often chosen to complement the overall aesthetic of the kimono or other garments worn.

In addition to securing the katana, the obi can also serve decorative purposes. It may feature intricate patterns, embroidery, or other embellishments that showcase the wearer's personal style or social status.

It's worth noting that there are different styles and types of obi, each suited for specific occasions or garments. However, when referring to the belt used specifically for a katana, it is commonly referred to as the obi.